Space shuttle exhaust reveals behavior of atmospheric winds

After the Space Shuttle Atlantis launched for the final time at 11:29 AM (EDT) on July 8, 2011, scientist tracked water vapor in its exhaust on its travels throughout the upper atmosphere. Credit: NASA Photo/Houston Chronicle, Smiley N. Pool

On
July 8, 2011 the Space Shuttle Atlantis launched for the very last
time. On that historic day, as the world watched its last ascent up into
orbit and commentators discussed the program's contributions to space
flight and scientific research over 20 years, the shuttle helped spawn
one last experiment. As the shuttle reached a height of about 70 miles
over the east coast of the U.S., it released—as it always did shortly
after launch—350 tons of water vapor exhaust.

As
the plume of vapor spread and floated on air currents high in Earth's
atmosphere, it crossed through the observation paths of seven separate
sets of instruments. A group of scientists, reporting in online in the
Journal of Geophysical Research on August 27, 2012, tracked the plume to
learn more about the airflow in the Mesosphere and Lower Thermosphere
(MLT)—a region that is typically quite hard to study. The team found the
water vapor spread much faster than expected and that within 21 hours
much of it collected near the arctic where it formed unusually bright
high altitude clouds of a kind known as polar mesospheric clouds (PMCs).
Such information will help improve global circulation models of air
movement in the upper atmosphere, and also help with ongoing studies of
PMCs.

"Polar
mesospheric clouds are the highest clouds on Earth," says space
scientist Michael Stevens at the Naval Research Laboratory, Washington,
who is first author on the paper. "They shine brightly when the sun is
just below the horizon and typically occur over polar regions in the
summer. There is some evidence that they are increasing in number and
people want to know if this is indicative of climate change or something
else that we don't understand."

click to enlarge

NASA's Aeronomy In the Mesosphere (AIM) mission captures images like this of shining noctilucent clouds, also known as polar mesospheric clouds (PMCs), which hover over Earth's poles in summertime. Credit: NASA/AIM

Since
they shine at night, PMCs are also known as noctilucent clouds, and
they can serve as an indicator not just of temperature changes, but also
of how currents and waves move high in Earth's atmosphere. A visible
cloud of water vapor from something like the shuttle also offers a
serendipitous way to observe such motions in the upper winds.

"The
plume from the shuttle becomes a ready-made experiment to observe the
movement in the atmosphere," says Charles Jackman, a scientist at NASA's
Goddard Space Flight Center in Greenbelt, Md. who is the project
scientist for a NASA mission called Aeronomy Ice in the Mesosphere (AIM)
that specifically observes PMCs. "What this team found is interesting
since the plume moved so quickly to the pole, indicating that the winds
appear much stronger at those latitudes than was thought."

To
track the plume across the sky, the scientists collated seven sets of
observations, including data from AIM. The first two sets of instruments
to see the plume were on a NASA spacecraft called TIMED (Thermosphere
Ionosphere Mesosphere Energetics and Dynamics). Next the plume was
viewed through the Sub-Millimeter Radiometer on the Swedish Odin
satellite. When the plume reached higher latitudes, it was picked up by
the ground-based Microwave Spectrometer at the Institute of Atmospheric
Physics in Kühlungsborn, Germany as well as an identical ground-based
water vapor instrument called cWASPAM1 at the Arctic Lidar Observatory
for Middle Atmospheric Research (ALOMAR) in Andenes, Norway. The plume
collated into its final shape over the arctic, as a new, extremely
bright PMC on July 9, 2011 and there, it could be observed from above by
the AIM satellite flying overhead, and from below by another instrument
at ALOMAR called the RMR lidar.

Over
the course of the plume's travels, these observations showed it
spreading horizontally over a distance of some 2000 to 2500 miles. Those
parts that drifted into the high latitudes near the North Pole formed
ice particles which settled into layers of PMCs down at about 55 miles
above Earth’s surface. The speed with which the plume arrived at the
arctic was a surprise.

click to enlarge

Noctilucent clouds – also known as polar mesospheric clouds (PMCs) over Kühlungsborn, Germany on July 9, 2011. These clouds shine brightly even during the night. Shuttle exhaust made of water vapor formed particularly bright PMCs on July 9, 2011 over Scandanavia. Credit: Leibniz-Institute of Atmospheric Physics

"The
speed of the movement in the upper atmosphere gives us new information
for our models," says Stevens. "As you get higher up in the atmosphere,
we just don't have as many measurements of wind speeds or temperatures.
The take-away message here is that we need to improve the models of that
region."

Since
observations of PMCs may be connected to global climate, it's important
to subtract out sporadic effects such as shuttle exhaust from other
consistent, long-term effects.

"One
of AIM's big goals is to find out how much of the cloud's behavior is
naturally induced versus man-made," says Jackman. "This last shuttle
launch will help researchers separate the shuttle exhaust from the rest
of the observations."

Indeed,
the AIM observations showed a clear difference between typical PMCs and
this shuttle-made one. Normally smaller particles exist at the top,
with larger ones at the bottom. The shuttle plume PMC showed a reversed
configuration, with larger particles at the top, and smaller at the
bottom—offering a way to separate out such clouds in the historical
record.